<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <button>按钮1</button>

<script>
 const but1= document.querySelector('button')
   but1.onclick = ()=>{
     alert('--111111--')
   }
   but1.onclick = ()=>{
     alert('--22222--')
   }
   but1.addEventListener('click',()=>{
     alert('--333333--')
   })
   but1.addEventListener('click',function(){
     alert('--444444--')
   })
   but1.addEventListener('mouseenter',)

</script>
</body>
</html>